The present invention relates to a transfer switch, and in particular to a transfer switch that limits contact closing speed.
A transfer switch is used to switch an electric load back and forth between a primary source, such as a utility, and a secondary source, such as a generator. Transferring power from the primary source to the secondary source is necessary when the incoming power quality deviates from set limits. The transfer switch is also used to switch the source back to utility power when the power quality returns to within the preset limit.
Some transfer switches have more control than others as they change power sources. Many transfer switches are able to disconnect the load from both sources for a desired period of time in order to allow residual electricity to discharge before the load is switched to an alternate power source.
A typical transfer switch includes a reciprocating toggle mechanism. The toggle mechanism includes contacts that move along with the toggle mechanism relative to stationary contacts on the transfer switch. The movable contacts engage one set of stationary contacts when power is supplied by the primary source and engage another set of contacts when power is supplied from the secondary source.
The toggle mechanism often includes a rotating crossbar such that the moving contacts are mounted on the crossbar. The crossbar is connected to springs that store energy within an actuation mechanism. The actuating mechanism is activated either manually or automatically at a desired time to release the stored energy and move the crossbar. The crossbar moves very fast such that the crossbar and contacts have a significant amount of kinetic energy as the moving contacts engage either set of stationary contacts. The toggle mechanisms and contacts in transfer switches with high short-circuit withstand capability are usually more massive such that these types of toggle mechanisms have even greater kinetic energy.
As the moving contacts engage the stationary contacts, the kinetic energy of the moving contacts causes the contacts on the crossbar to bounce up and down on the stationary contacts until the kinetic energy is dissipated. Contact bounce can cause arcing that damages the contacts. When there is contact arcing at high current, the contacts can be severely eroded or even completely vaporized.
The high speed of the moving contacts can also cause the contacts to crack as they impact the stationary contacts, especially when the contacts within the transfer switch are massive. The sudden deceleration of the toggle mechanism can also cause components with the toggle mechanism to bend or break.
The present invention relates to a transfer switch that absorbs the kinetic energy of a toggle mechanism within the transfer switch just before moving contacts on the toggle mechanism engage a set of stationary contacts. The moving contacts travel at high speed as they move toward the stationary contacts, and just before the moving contacts engage the stationary contacts an energy absorbing device removes the kinetic energy from the toggle mechanism. Reducing the kinetic energy of the moving contacts prior to engaging the stationary contacts minimizes contact bounce, especially when the transfer switch includes massive contacts, such as those used in transfer switches having high short-circuit withstand and closing capability.
The transfer switch includes output contacts, primary input contacts, secondary input contacts and a toggle mechanism. The toggle mechanism includes moving contacts that alternately connect the output contacts with the primary and secondary input contacts. The transfer switch further includes a dampener that is connected to the toggle mechanism. The dampener reduces the kinetic energy of the moving contacts before the moving contacts engage the input contacts.
The present invention also relates a method of alternating the supply of power to an electric load. The method includes switching contacts within a transfer switch to alternately engage the switching contacts with input contacts that are connected to a primary power source and input contacts that connected to a secondary power source. The method further includes reducing the kinetic energy of the switching contacts before the switching contacts engage the input contacts.